A seed is essentially a tiny plant embryo awaiting the proper amount of moisture, oxygen, temperature and light. When the environment meets the proper conditions, the seed germinates, which means it begins to sprout and grow into a seedling.
A seed is alive, but is simply suspended until germination. The bulk of a seed is a starchy substance; it breaks down and is used as fuel by the baby plant until it gets enough sunlight to sustain itself. There is a tiny plant embryo somewhere in every seed, and just adding water is often enough to trigger growth and germination. The environmental conditions don't have to be exact, so germination can begin in a variety of conditions depending on the seed type and location.